1. Field of the Invention
This invention is related to the fabrication of fiber probes for use in near-field scanning microscopy. More specifically, this invention is related to a focused ion-beam fabrication method for well-defined subwavelenth fiber probes with metallic apertures.
2. Related Art
Near-field scanning optical microscopy (NSOM) is capable of subwavelength optical resolution by breaking the diffraction limit of conventional optical microscopy. The optical near field of a source exists within a distance of less than one optical wavelength from that source. By placing an aperture of much less than the optical wavelength in a metal screen placed in near-field proximity to a sample, NSOM can achieve a resolving power below that of classical optical microscopy.
Conventional practical implementation of such an aperture has been accomplished by first tapering down an optical fiber to subwavelength diameter. Such tapering can be performed, for example, through a heating-pulling technique. In such a technique, a fiber is put under tension while heat is applied. The heat can be from a CO2 laser, for example. As the fiber heats under tension it can be separated into two pieces, each having a tapered end. Modifying the tension used, and the amount of heat applied, determines the size and shape of the taper.
Alternatively, chemical etching can be used to form an optical fiber with a tapered end. In such a technique, an optical fiber is immersed in an acid such as hydrofluoric acid. If this is done appropriately, the result is a tapered tip. As with the heating-pulling method, the size and shape of the fiber can be altered during the chemical etching technique. For example, the speed at which the fiber is pulled from the acid can be modified so as to change the shape of the tapered region.
A hybrid pull-etch technique can also be used to form tapered fibers. Such a technique involves both heating-pulling as well as chemical etching. Tapered regions are first formed with the heating-pulling technique, discussed above. Next, the tapered-tips of the fibers are subjected to chemical etching.
Once a tapered fiber is produced in accordance with a technique like those discussed above, or otherwise known to one skilled in the relevant art, an angle-evaporation process is preformed to provide a partial metal coating on the fiber. This partial metal coating covers the sidewalls of the tapered fiber, resulting in an aperture located at the end of the fiber. The process is conducted through vacuum deposition of aluminum, gold, chromium, or another appropriate metal by thermal evaporation while the tip of the fiber is rotated and held at an angle to the source.
The exposed fiber tip created through the above process is generally located inside a metallic aperture that is partially obstructed by rough aluminum grains situated near the aperture boundary. Frequently, such an aperture contains aluminum protrusions, contributing to the poor polarization capabilities common to fiber tips produced through conventional methods. Likewise, these defects can also result in a larger than optimal separation of fiber tip and sample as well as the absence of a clean opening, which can lead to lower throughput of the probe and poor optical resolution of the NSOM. Furthermore, the angle-evaporation process is very time consuming and typically does not result in the fabrication of a batch of reproducible and well-defined apertures of desired diameter. Moreover, other problems associated with angle evaporation are the fabrication of a special holder to accommodate many fibers with adjustable tilt angle, the loading of the probes, and the difficulty associated with keeping the fibers untangled throughout the procedure.
What is needed is a method of producing clean, well-defined, and highly reproducible subwavelength apertures for NSOM that minimizes those problems associated with the conventional techniques discussed above.
In an embodiment of the instant invention, a method of forming a fiber probe having an aperture for use in near-field scanning optical microscopy is disclosed. The disclosed method includes a first step of coating an optical fiber having a tapered tip with a metal layer. The tapered tip can be formed with a heating-pulling process, an etching process, or a combination of the two.
Once the optical fiber is coated with the metal layer, there is a step of milling the tapered tip and metal layer such that an aperture is formed through the metal layer at the tapered tip. The coated metal layer can be an aluminum, gold, or chromium layer. The metal layer can be a sputtered layer having a thickness of less than 150 nanometers, preferably between 20 and 150 nanometers, and more preferably between 100 and 150 nanometers.
The milling step includes focused ion-beam milling the tapered tip and metal layer and can be preceded by a step of imaging the tapered tip with an electron beam. The focused ion-beam milling can be done by raster scanning the focused ion-beam in a rectangular pattern at an apex of the tapered tip such that portions of the metal layer and tapered tip are removed at the apex, forming an aperture through the metal layer. An overlap of the rectangular pattern and the apex of the tapered tip is chosen so as to produce an aperture having a predetermined diameter. The predetermined diameter can be less than a wavelength of light used in the near-field scanning optical microscopy. Preferably, the predetermined diameter can be approximately 100 nanometers or less.
Also disclosed is a fiber probe for use in near-field scanning optical microscopy made through the above outlined method. Thus, the fiber probe according to an embodiment of the invention includes an optical fiber having a tapered tip, a metal layer coated on said optical fiber at the tapered tip, and a focused ion-beam milled aperture through the metal layer at an end of the tapered tip. The metal layer can be an aluminum layer, a chromium layer, or a gold layer having a thickness of less than 150 nanometers, preferably between 20 and 150 nanometers, and more preferably between 100 and 150 nanometers.
The fiber probe of the present invention can have a focused ion-beam milled aperture with a diameter less than a wavelength of light used in the near-field scanning optical microscopy. Preferably, the diameter is approximately 100 nanometers or less. Furthermore, the optical fiber and the metal layer have co-planar exterior end surfaces at a terminal end of said tapered tip, with the exterior end surface of the metal layer surrounding the exterior end surface of the optical fiber.